#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>
#include <vector>

using std::vector;

struct point
{
   double x, y;
};

double area(const vector<point>& a)
{
   double s = 0;
   for (size_t i = 0; i < a.size(); ++i)
      s += a[i].x * a[(i + 1) % a.size()].y - a[(i + 1) % a.size()].x * a[i].y;
   s *= 0.5;
   return s;
}

int main()
{
   std::ifstream input ("in.txt");
   std::ofstream output ("out.txt");
   int i = 0;
   double x,y;
   vector <point> a;
   while (input >> x >> y)
   {
      point q;
      q.x = x;
      q.y = y;
      a.push_back(q);
   }
   output << std::fixed;
   output.precision(7);
   output << abs(area(a));
   return 0;
}

